1. Field of the Invention
This invention relates to a curing apparatus suitable for the production of shaped articles of cross-linked polymeric material such as cross-linked polymeric material insulated wires, and more particularly to a curing apparatus in which a curing tube is heated by the direct passage of electric current in the tube.
2. Description of the Prior Art
It is already known to cross-linked a polymeric material by passing the material through a curing tube with a.c. or d.c. voltage applied across the terminals at the opposite ends of the tube and heating the material with the heat resulting from the electric resistance of the tube as disclosed in U.S. Pat. Nos. 2,616,126; 2,581,769, etc. This method of heating is more advantageous in equipment cost and operation procedure than other methods in which the curing tube is heated with a hot fluid circulating through a jacket as disclosed in U.S. Pat. No. 3,054,142 or with a band heater as used in U.S. Pat. No. 3,928,525. The method, nevertheless, involves the problem that the uncured polymeric material continuously fed at a low temperature (usually 100.degree. to 130.degree. C.) to the curing tube during curing operation absorbs part of the heat of the curing tube, reducing the temperature of the curing tube at and near its inlet. Such a reduction in the temperature of the curing tube will retard the cross-linking reaction velocity of the polymeric material, consequently leading to a reduced production efficiency. When the curing tube is a so-called long-land die, the polymeric material to be cross-linked advances along the inner surface of the long-land die with a thin layer of the lubricant provided therebetween while absorbing an especially large amount of heat, possible decreasing the temperature of the die at its inlet to a level nearly 80.degree. C. lower than the highest temperature at the outlet thereof. Although it appears possible to compensate for the drop in the curing temperature attendant on the conventional heating method of the above voltage application type by impressing higher voltage across the terminals to ensure increased heat generation, this is practically infeasible since the greatly elevated temperature in the vicinity of the tube outlet is likely to thermally degrade the polymeric material.
The curing tube, which is as long as several meters to several tens of meters, is usually composed of a number of short tubular segments connected together in series. If the tube involves a difference in electric resistance among tubular segments or an insufficient electrical connection at the end-to-end joint between segments, application of voltage would produce a low-temperature portion or an abnormally hot portion longitudinally of the tube. Such a low-temperature portion, if created, also would reduce the operation efficiency.